Process for the preparation of high-purity magnesium hydroxide and magnesium oxide from magnesium alkoxides

ABSTRACT

There is provided a process for producing high-purity magnesium hydroxide by reaction of magnesium or reactive magnesium compounds with hydroxy compounds yielding magnesium alkoxides, followed by hydrolysis to form magnesium hydroxide, or a process for producing magnesium oxide by calcination of magnesium hydroxide.

[0001] The present invention relates to a process for producing high-purity magnesium hydroxide and for obtaining therefrom magnesium oxide by calcination.

[0002] Natural resources of magnesium hydroxide [CRN 1309-48-4] are rare so that this material is seldom mined. Nowadays, magnesium hydroxide is obtained by precipitation from sea-water (cf. Gilpin, W. C. and Heasman, N., Chem. Ind. (London), 1977, p. 567-572; Drum, J. C., Tangney, S., Trans. J. Br. Ceramic Soc., 77 (1978), no. 4, p. 10-14) and precipitation from magnesium salt solutions using calcium hydroxide.

[0003] Said manufacturing processes have one disadvantage: the magnesium hydroxide produced in this way is hardly suitable for a large number of catalytic operations and for the production of special ceramics. This is primarily due to impurities in the form of other metals making the magnesium hydroxide particularly unsuitable for catalytic processes.

[0004] DE 3 244 972-C1 discloses a continuous process for producing high-purity-aluminium alcoholates. According to said publication, aluminium metal is reacted with alcohol yielding aluminium alcoholate which can be liberated from other metals present in the aluminium metal by filtration and/or distillation because said metals are not converted or are only slowly converted into metal alcoholates. Up to now, however, said process has not been applicable to magnesium because the magnesium alcoholates known in the art are not liquid and, therefore, cannot be filtered.

[0005] Furthermore, they cannot be melted without undergoing decomposition and, consequently, cannot be distilled.

[0006] EP 0 244 917 describes a process for producing soluble metal alkoxides from alkoxy alcohols in organic solvents, but no reference is made therein to a process for the production of high-purity, crystalline magnesium hydroxides having fine porosities and uniform crystallinities.

[0007] Therefore, it was the object of this invention to develop a process for producing magnesium hydroxide having the following features:

[0008] The magnesium hydroxide produced according to the invention is required to have high purity, fine porosity, and uniform crystallinity.

[0009] The starting materials shall be inexpensive and readily available.

[0010] The manufacturing process shall be feasible both continuously and discontinuously.

[0011] Surprisingly, it has been found that the problems cited hereinabove can be solved when employing the process described hereinbelow.

[0012] The present invention refers to a process for the continuous or discontinuous production of high-purity magnesium hydroxide and/or magnesium oxide by reacting magnesium metal and reactive magnesium compounds with hydroxy compounds of the type R¹—A—R—OH and hydrolysing the resultant product, optionally followed by calcination if it is desirable to produce magnesium oxide. Optionally, the reaction is performed using also up to 50 percent by weight alcohols of the type R2—OH.

[0013] The hydroxy compounds to be used according to the present invention have the general formula R¹—A—R—OH, wherein A represents an element of main group 6 of the periodic system (starting with oxygen) or main group 5 of the periodic system (starting with nitrogen). If A represents the nitrogen group, A may bear additional substituents, preferably a total of three, for saturation of its valences. These substituents can be hydrogen or an additional hydrocarbon residue R¹ which is optionally different from the other ones. It is preferred that the heteroatoms A be oxygen and nitrogen, oxygen being particularly preferred.

[0014] R, R¹, and R² represent a branched or unbranched, cyclic or acyclic, saturated, unsaturated, or aromatic hydrocarbon residue having 1 to 10 carbon atoms, wherein R, R¹, and R² may be different from each other and R is twofold substituted (divalent).

[0015] Preferably, R¹ is a saturated alkyl residue having 1 to 10 carbon atoms, particularly 1 to 5 carbon atoms, the alkyl residue being most preferably unbranched. It is preferred that R² be a branched or unbranched, cyclic or acyclic, saturated hydrocarbon residue having 4 to 8 carbon atoms, the hydrocarbon residue being most preferably unbranched, acyclic, and saturated. Preferably, R are branched or unbranched, acyclic alkylidene residues having 1 to 5 carbon atoms, unbranched hydrocarbons having 1 to 3 carbon atoms being particularly preferred.

[0016] The instant invention is based on a process for the continuous or discontinuous production of liquid magnesium alkoxides by reaction of suitable hydroxy compounds with magnesium compounds and/or magnesium metal which are reactive for said hydroxy compounds. For example, a suitable reactive magnesium compound is magnesium hydride. Magnesium metal is a particularly preferred material. It was surprisingly found that when reacting organic compounds of type R¹—A—R—OH with magnesium or reactive magnesium compounds, the resultant magnesium alkoxides are already liquid at room temperature.

[0017] The present invention also comprises the continuous or discontinuous hydrolysis of said liquid magnesium alkoxides, particularly magnesium alkoxy ethers and magnesium alkoxy amines, which is carried out after difficultly soluble impurities have been separated, e.g. by filtration, centrifugation, or decantation, for preparing high-purity magnesium hydroxide. After hydrolysis, there is achieved good phase separation between the water/magnesium hydroxide mixture and the alcohol components, when compounds which are free from foreign metal ions and which are suitable for salting-out, particularly 0.1 to 10 percent by weight ammonium hydrogen carbonate are added to the water for hydrolysis.

[0018] The hydroxy compounds of type R¹—A—R—OH referred to hereinabove can be used alone (one compound) or as a mixture (several compounds of type R¹—A—R—OH). Particularly if higher-viscous compounds are formed by the reaction of this invention, sedimentation of impurities which are difficultly soluble solids is impeded. In this case, the hydroxy compounds may be used in a one- to fivefold excess in order to reduce the viscosity and thus enhance filterability. For this purpose also alcohols of type R²—OH can be used or can be used in larger quantities. The viscosity of the liquid/solution is most preferably adjusted by subsequent addition of R²—OH alcohols.

[0019] Prior to or after reaction, part of the R¹—A—R—OH hydroxy compounds used for the reaction can be substituted for other alcohols, i.e. those of the R2—OH type. However, the total amount of said R²—OH alcohols must not be higher than 50 percent by weight, referring to the total quantity of educts and solvents to be used.

[0020] In principle, the reaction of hydroxy compounds with magnesium compounds can also be carried out using solvents. However, use of such solvents will involve higher production cost because they must be removed after conversion and, what is even more important, foreign solvents will have an adverse effect on the material properties of magnesium hydroxides, i.e. purity, uniform pore distribution, and crystallinity. Furthermore, when using foreign solvents, magnesium compounds often become solid and insoluble after conversion and purification making subsequent dissolution in a nonpolar solvent necessary before the solution can be subjected to hydrolysis. When using other solvents for the reaction, amorphous magnesium hydroxides are obtained.

[0021] Magnesium alkoxy compounds which are suitable for the production of high-purity magnesium hydroxide according to this invention include for example the experimental products bis(ethylglycolato)-magnesium (VP 1), bis(n-butyl-glycolato)-magnesium (VP 2), magnesium-bis(N,N-dimethyl-amino-1-propanolate) (VP 3), magnesium-bis(N,N-dimethyl-aminoethanolate) (VP4), magnesium-bis(2-ethylaminoethano-late) (VP 5), or magnesium-bis(1-methoxy-2-propanolate) (VP 6).

[0022] The magnesium hydroxide produced according to this invention has high purity. In particular, the alkali and alkaline-earth metal contents which have particularly unfavorable effects in catalytic applications are very low. The results of trace elements determinations by ICP are listed in Table 1. For comparison, premium-grade magnesium hydroxide of highest purity and reagent-grade magnesium oxide, both commercially available, were included in the examinations. TABLE 1 Trace Elements Determinations by ICP Fe Si Ti Mn Zn Ga Na Ca Compound [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] Magnesium 286 46 3 8 23 <5 23 9 Mg-alkoxide <2 <2 <1 <1 <2 <5 <2 <1 Mg(OH)₂ a 12 <2 3 3 <2 <5 14 6 Mg(OH)₂ b 52 699 1 7 2 <5 58 2,290 MgO c 9 51 1 6 2 4 1,375 168 MgO d 15 7 3 3 <2 <5 16 10

[0023] When comparing the impurities in the magnesium metal and the impurities in the magnesium alkoxide compound prepared therefrom, it becomes apparent that the separation of impurities, e.g. other metals, is performed in a highly efficient way. High-purity magnesium oxides or hydroxides within the meaning of the present invention are those products reaching the threshold values listed in Table 2. TABLE 2 Threshold Values Fe Si Ti Mn Zn Ga Na Ca Compound [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] <50 <50 <10 <10 <10 <10 <50 <50

[0024] In particular, the threshold values listed in Table 3 characterise high-purity products, the low concentrations of alkali ions and alkaline-earth ions, particularly Na and Ca, being most essential and making said materials very useful for a large number of catalytic applications which are very sensitive to alkali and alkaline-earth foreign ions. TABLE 3 Threshold Values Fe Si Ti Mn Zn Ga Na Ca Compound [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] <20 <20 <5 <5 <5 <5 <20 <20

[0025] The purities shown in Table 1 can be further increased by using bidistilled water and containers made of inert materials. When using deionised water for the hydrolysis, the Fe, Mn, Ti, Na, and Ca concentrations in Mg(OH)₂ (compound a) slightly increase as compared with the magnesium alkoxide feedstock (Mg-alkoxide).

[0026]FIG. 1 represents the X-ray diffraction pattern of magnesium hydroxide prepared according to this invention. For comparison, the diffraction pattern of the JCPDS file (no. 7-0239, Mg(OH)₂, brucite, syn) is shown as well.

[0027] Metal oxides can be obtained by calcination of the compounds prepared according to this invention. The compounds of this invention were calcined in a furnace at temperatures of 550 to 1,500° C. for a period of 3 to 24 hours. The metal oxide prepared in this way has the same high purity as the metal hydroxide of this invention.

[0028] In Table 4 several physical data of the magnesium hydroxide prepared according to this invention have been compiled. TABLE 4 Physical Data of Experimental Product VP 1 Temperature H₂O: Pore Water for of Water for Alcoholate Surface Pore Volume Radius Hydrolysis Hydrolysis [g/g] [m²/g] [ml/g] [Å] [pH] [° C.] 1.15:1 142 0.71¹⁾ 94 7 90 2:1 123 0.62¹⁾ 68 7 90 4:1 142 0.62²⁾ 88 7 90 4:1 97 0.76¹⁾ 283 1 90 4:1 131 0.40¹⁾ 78 4 90

[0029] Based on the assumption that the pore radius correlates with the crystallite size, it becomes apparent that the desired pore radius and, thus, the desired crystallite size of the magnesium hydroxide of this invention can be obtained by adjusting the pH-value of the water for hydrolysis, e.g. by addition of ammonia to the water for hydrolysis.

[0030] The magnesium hydroxides and oxides of this invention have significantly narrower and more uniform pore distributions (monomodal distributions) than conventional, commercially available products of this kind. For comparison, the pore distributions of magnesium hydroxide and oxide are shown in FIGS. 2 and 3. Precisely defined pore radii and narrow pore distributions have great importance in catalytic applications.

[0031] The trace impurities in the compounds of this invention were determined by inductively coupled plasma spectroscopy (ICP), while the crystalline phases were determined by powder diffractometry. The surfaces were determined by the BET method, while pore volumes and radii were additionally determined by mercury porosimetry and nitrogen porosimetry. The compounds of this invention were calcined in a muffle furnace at temperatures of between 550° C. and 1,500 ° C. Deionised water was used for the hydrolysis.

EXAMPLE 1

[0032] Reaction with Ethyl Glycol (Stoichiometric Amounts of CH₃—CH₂—O—CH₂—CH₂—OH); ammoniacal hydrolysis (VP 1)

[0033] Into a 1,000-ml three-neck flask, there were placed 20 grams of granular magnesium to which 48.4 grams of ethyl glycol were added. The mixture was heated. Reaction of the metal with ethyl glycol started at approx. 125° C. (perceptible by the formation of hydrogen and a temperature increase to approx. 140° C.). Then, 100 grams of ethyl glycol were added during a period of 30 minutes using a dropping funnel. The liquid reaction mixture was filtered, and 100 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 400 grams of deionised water containing 0.2 percent by weight ammonia (T=90° C.). Hydrolysis resulted in immediate formation of white precipitate. The released alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 98% of theoretical.

Example 1a

[0034] Reaction with Ethyl Glycol (VP 1)

[0035] Into a 1,000-ml three-neck flask, there were placed 20 grams of granular magnesium to which 50 grams of ethyl glycol were added. The mixture was heated. Reaction of the metal with ethyl glycol started at approx. 125° C. (perceptible by the formation of hydrogen and a temperature increase to approx. 140° C.). Then, 236 grams of ethyl glycol were added during a period of 50 minutes using a dropping funnel. The reaction mixture was filtered, and 100 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 400 grams of deionised water containing 0.2 percent by weight ammonia (T=90° C.). A white precipitate formed immediately. The released alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 98% of theoretical.

EXAMPLE 2

[0036] Reaction with Ethyl Glycol and Hexanol; Ammoniacal Hydrolysis (VP 1)

[0037] Into a 1,000-ml three-neck flask, there were placed 20 grams of granular magnesium to which 20 grams of a hexanol/ethyl glycol mixture (50:50 percent by weight) were added. The mixture was heated. Reaction of the metal with the hexanol/ethyl glycol mixture started at approx. 125° C. (perceptible by the formation of hydrogen and a temperature increase to approx. 140° C.). Then, 266 grams of the hexanol/ethyl glycol mixture were added during a period of 50 minutes using a dropping funnel. The reaction mixture which was still liquid at room temperature was filtered, and 100 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 400 grams of deionised water containing 0.2 percent by weight ammonia (T 90° C.). A white precipitate formed immediately. The released alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 98% of theoretical.

EXAMPLE 3

[0038] Reaction with N-butyl Glycol (CH₃—CH₂—CH₂—CH₂—O—CH₂—CH₂—OH); Ammoniacal Hydrolysis (VP 2)

[0039] Into a 1,000-ml three-neck flask, there were placed 15 grams of granular magnesium to which 15 grams of n-butyl glycol were added. The mixture was heated. Reaction of the metal with n-butyl glycol started at approx. 150° C. (perceptible by the formation of hydrogen). The reaction temperature increased to approx. 180° C. Then, 252 grams of n-butyl glycol were added during a period of 90 minutes using a dropping funnel. The reaction mixture was filtered, and 100 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 400 grams of deionised water containing 0.2 percent by weight ammonia (T=90° C.). A white precipitate formed immediately. The released alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 98% of theoretical.

Example 3a

[0040] Reaction with N-butyl Glycol (Stoichiometric Amount); Hydrolysis Without Ammonia (VP 2)

[0041] Into a 1,000-ml three-neck flask, there were placed 15 grams of granular magnesium to which 25.7 grams of n-butyl glycol were added. The mixture was heated. Reaction of the metal with n-butyl glycol started at approx. 155-160° C. (perceptible by the formation of hydrogen). Then, 120 grams of n-butyl glycol were added during a period of 90 minutes using a dropping funnel. The liquid reaction mixture was filtered, and 100 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 400 grams of deionised water (T 32 90° C.). A white precipitate formed immediately. The released alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 98% of theoretical.

Example 3b

[0042] Reaction with N-butyl Glycol; Hydrolysis Without Ammonia (VP 2)

[0043] Into a 1,000-ml three-neck flask, there were placed 15 grams of granular magnesium to which 15 grams of n-butyl glycol were added. The mixture was heated. Reaction of the metal with n-butyl glycol started at approx. 155-160° C. (perceptible by the formation of hydrogen). Then, 252 grams of n-butyl glycol were added during a period of 90 minutes using a dropping funnel. The reaction mixture was filtered, and 100 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 400 grams of deionised water (T=90° C.). A white precipitate formed immediately. The supernatant alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 98% of theoretical.

EXAMPLE 4

[0044] Reaction with 3-dimethylamino-1-propanol (VP 3)

[0045] Into a 1,000-ml three-neck flask, there were placed 15 grams of granular magnesium to which 50 grams of 3-dimethylamino-1-propanol were added. The mixture was heated. Reaction of the metal with 3-dimethylamino-1-propanol started at approx. 150-160° C. (perceptible by the formation of hydrogen). Then, 197 grams of 3-dimethylamino-1-propanol were added during a period of 6 hours using a dropping funnel. The reaction mixture was filtered, and 100 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 400 grams of deionised water (T=90° C.). A white precipitate formed immediately. The released alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 95% of theoretical.

EXAMPLE 5

[0046] Reaction with 2-(Dimethylamino)Ethanol (VP 4)

[0047] Into a 1,000-ml three-neck flask, there were placed 15 grams of granular magnesium to which 50 grams of 2-(dimethylamino)ethanol were added. The mixture was heated. Reaction of the metal with 2-(dimethylamino)ethanol started at approx. 135-145° C. (perceptible by the formation of hydrogen). Then, 321 grams of 2-(dimethylamino)ethanol were added during a period of 5 hours using a dropping funnel. The reaction mixture was filtered, and 100 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 400 grams of deionised water (T=90° C.). A white precipitate formed immediately. The released alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 96% of theoretical.

EXAMPLE 6

[0048] Reaction with 2-ethylamino-ethanol (VP 5)

[0049] Into a 1,000-ml three-neck flask, there were placed 15 grams of granular magnesium to which 50 grams of 2-ethylamino-ethanol were added. The mixture was heated. Reaction of the metal with 2-ethylamino-ethanol started at approx. 150-160° C. (perceptible by the formation of hydrogen). Then, 123 grams of 2-ethylamino-ethanol were added during a period of 4 hours using a dropping funnel. The reaction mixture was filtered, and 100 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 400 grams of deionised water (T=90° C.). A white precipitate formed immediately. The released alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 94% of theoretical.

EXAMPLE 7

[0050] Reaction with 1-(Methoxy)Propan-2-ol (VP 6)

[0051] Into a 500-ml three-neck flask, there were placed 10 grams of granular magnesium to which 15 grams of 1-(methoxy)propan-2-ol were added. The mixture was heated. Reaction of the metal with 1-(methoxy)propan-2-ol started at approx. 120° C. (perceptible by the formation of hydrogen). Then, 59.1 grams of 1-(methoxy)propan-2-ol were added during a period of 5 hours using a dropping funnel. The reaction mixture was filtered, and 50 grams of the filtrate divided into three aliquot portions were hydrolysed in a H₂O/alcoholate mixture (4:1) consisting of 200 grams of deionised water (T=90° C.). A white precipitate formed immediately. The released alcohol was distilled off, and the resultant suspension was spray dried. The yield was equal to 80% of theoretical. 

1. A process for producing high-purity magnesium hydroxide characterized in that magnesium metal and/or reactive magnesium compounds are reacted with one or more hydroxy compound(s) of the type R¹—A—R—OH  (I) optionally together with up to 50 percent by weight of one or more alcohol(s) of the type R²—OH  (II) to form liquid magnesium alkoxides, and that, optionally, less soluble impurities are separated and hydrolysis is performed, wherein (a) A represents an element of main group 6 (oxygen group) or main group 5 (nitrogen group) of the periodic system, wherein if A represents an element of main group 5, A bears additional substituents R¹ or hydrogen which may be different from each other; and (b) R, R¹ and R² represent a branched or unbranched, cyclic or acyclic, saturated, unsaturated, or aromatic hydrocarbon residue having 1 to 10 carbon atoms, wherein R, R¹, and R² may be different from each other and R is twofold substituted.
 2. A process according to claim 1, characterized in that magnesium metal is used.
 3. A process according to any one of the preceding claims wherein R and R^(1,) represent a branched or unbranched, saturated hydrocarbon residue having 1 to 5 carbon atoms and R and R¹ may be different from each other and R is twofold substituted.
 4. A process according to any one of the preceding claims, characterised in that R² represents an unbranched, acyclic and saturated hydrocarbon residue having 4 to 8 carbon atoms.
 5. A process according to any one of the preceding claims, characterised in that the hydroxy compounds are employed in a one- to fivefold excess, referring to the stoichiometric ratio of hydroxy groups to Mg valences.
 6. A process according to any one of the preceding claims, characterised in that the reaction is essentially exclusively carried out, without addition of solvents, with/in the hydroxy compounds of type I or II to form magnesium hydroxides.
 7. A process according to any one of the preceding claims, characterised in that the hydroxy compounds of type II are added not until after the reaction of the metal/metal compound with the hydroxy compounds of type I.
 8. A process according to any one of the preceding claims, characterised in that, prior to hydrolysis, the resultant magnesium alkoxy compounds are separated from the less soluble impurities.
 9. A process for producing magnesium oxides characterised in that the magnesium hydroxides according to any one of the preceding claims are calcined.
 10. The use of the magnesium hydroxide or magnesium oxide manufactured according to any one of claims 1 to 9 as a catalyst and/or catalyst support for catalytic processes, or for the manufacture thereof.
 11. The use of the magnesium hydroxide or magnesium oxide manufactured according to any one of claims 1 to 9 as a starting material for the production of ceramics or ceramic mouldings. 